Protective device



1943. s. R. SMITH, JR, ETAL 2,337,353

PROTECTIVE DEVICE Filed Dec. 26, 1941 NORMAL LIQUID DIELECTRIC LEVEL l I I I n a h m i u )NSULATING 3 COMPOUND Ies R Craig, e 7% 6 H15 Attorney Inventor:- Sidney RSmith Jr:

Char

Patented Dec. 21, 1943 UNITED STATES PATENT OFFICE PROTECTIVE DEVICE v Sidney a. Smith, Jr. and Charles-R. m Pittsfield, Mass, assignors to General Electric Company, a corporation-0t New York Application December 26, 1941, ScriaLNo. 424,430

(01. zoo-127i 8 Claims.

Our invention relates to a protective device and more particularly to a fused protective device for protecting electrical apparatus, such as' transformers and the like, which may be of the liquid insulation filledtype.

It is an object of our invention to provide a new and improved fused protective device for electrical apparatus capable of being mounted within a bushing of such apparatus.

It is another object of our invention to provide a. new and improved fused device and mounting therefor.

Further objects and advantages of our invention will become apparent as the following description proceeds and the features 'of novelty which characterize our invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

For a better understanding of our invention, reference may be had to the accompanying drawing in which Fig. 1 is a partial view of one application of our invention to. an electrical apparatus, such as a transformer, Fig. 2 is an enlarged sectional View of the bushing of the transformer of- Fig. 1 illustrating an application of the fused protective device of-our invention, Fig. 3

is a sectional view of the fused protective device of Fig. 2, and Fig. 4 is a view similar to Fig. 3 of a modification of our invention.

Although our invention may be employed in the protection of forms of electrical apparatus other than transformers, we have elected for the purpose of describing and illustrating our invention to show it as-applied to the protection of an electric translating apparatus, such. asa transformer.

In Figs. 1 and 2 of the drawing, Ill-designates the casing of an electric translating apparatus, such as a transformer, which may contain the usual windings and insulating liquid dielectric. Casing l0 may be provided with a cover ll suitably fastened thereto. Supported on cover II and extending from the top' thereof is at. least one insulated high-voltage bushing 12 through which electrical connections with the high-voltage windings of the transformer may be made. The bushing l2 includes a, hollow central-portion or chamber l3 through which the highvoltage lead extends so that connectionsmay be made with an external electrical circuit. The bushing i2 is preferably constructed-of a ceramic insulating material and includes a depending portion H which extends for some distance beneath the cover I I of casing ill.

The chamber iii of bushing l2 generally ac- Ill commodates an insulated lead. However, since distribution transformers often require fusedprotective devices, we have provided a fused, protective device generally indicated at 15V for insertion in the hollow central portion or chamber l3 of the bushing (2 in place ofjthe insulated lead.

Fuses have been used inside hollow bushings heretofore but difliculties in regard to corona.

vradio interference, and. improper operation onv operation is obtained on low currents as well as on high currents'and, furthermore, by greatly reducing the length of the fuse tube having the small bore, a marked improvement in the abili of the fuse tube to vent the gases produced'an'd thus avoid high tube pressure is obtained. We have discovered that it is desirable for the'fuse tube to have a bore of /8 of an inch or less. In terms of this bore, the length of the fuse tube and, consequently, the length of the fuse should satisfy the following-equation:

where L is the length in inches of the fuse tune or that portion of the fuse tube having the small bore, V is the operating voltage of the fuse device, d is the bore diameter in inches, and K is a constant which falls within the range of value between 200 and 400. With a fuse tube having such dimensions, we have found that a very marked increase in efilciency under all operating conditions i obtained.

In order to obtain satisfactory expulsion action, it is desirable that the inner surface of the fuse tube be constructed of a gas-evolving insulating material suchas fiber. However, it is mechanically difficult to construct a fiber tube having a fairly large outside diameter which has abore A; of an inch or less and, accordingly, in Fig. 3, we have illustrated a fiber tube l6 having a small bore of /8 of an inch or less and a length which satisfies the equation set forth above constructed of gas-evolving insulating material or the like. This tube of small bore is preferably mounted within a larger tube I! which may also be constructed of fiber or any other suitable insulating material and of a size to insure a tight fit betweeninsulating tubes l6 and H. The outside surface of tube It: is preferably knurled, whereby variations in the outside diameter of small bore tube 16 and the insidediameter of tube I! may readily be. absorbed to provide a tight press fit so that tubes l6 and I1 form substantially an integral structure. It should be understood, of course, that the fuse tube comprising portions l6 and I1 may be constructed of a single member if mechanically feasible. v

As shown in Fig. 3, theupper .end of outer tube I1 is preferably threaded to receive a thread ed upper fuse terminal l8 including a fusereceiving recess l9 and a flexible cable-receiv ing recess 20. The lower terminal of the fuse tube comprising members l8 and H is illustrated ascomprising acup-shaped conducting member 2| suitably fastened to the lower end edge of the hole in cover H are controlled so as of the fuse tube and having an opening 22 therein which is sufficiently large to permit passage therethrough of the fuse wir 23 which may have one end thereof soldered into fusereceiving recess IQ of terminal 18 while the other end thereof is soldered to the exterior of lower cup-shaped terminal 2|: -The opening 22 also permits gases to escape therethrough and. under severe fault conditions, cap 21 may be blown completely away from the fuse tube comprising members l6 and I1.-

Lower fuse terminal Zl-may be suitably connected to the transformer windings as by insulated conductor 23a. A suitable flexible cable 24 is preferably fastened to terminal I? as by being soldered into cable-receiving recess-20 and is electrically connected to a bolt 25 which is adapted to extend through a small opening 26 in the upper part of bushing l2 with which electrical connections may be made as by nut 21 and 28. Bolt 25 is preferably provided with a pair of lugs 29 for engaging with cooperating notches 38 formed in bushing l2 to prevent turning of bolt 25 while nuts 2'! and 28 are tightened thereon. Flexible cable 24 is preferably covered with insulation 31 as shown in Figs. 2 and 3.;

In order to minimiz both radio interference in the range of transformer voltage rating and corona discharge when the fuse device i subjected to high-voltage impulses, we have provided a wrapping of absorbent insulating material 32,

such as crepe paper or the like, for the entire fuse tube and flexible lead as shown in Figs. 2 and 3 so that, when the lower portion of the fuse tube is exposed to an insulating liquid dielectric. such as oil, the entire space 13 within the bushing 12 is in effect surrounded by such liquid dielectric due to capillary action even though the 1 main body of the fuse device is located above --the liquid dielectric level in the electric translatingapparatussuch as the transformer. As

shown in Fig. 2, preferably the crepe paper wrapping 32. extends somewhat below the depending portion l4 ofthe bushing to insure that it is partly immersed in the liquid dielectric even if the dielectric level should fall somewhat and also better to insulate the metal parts at the lower end of the fuse device. This arrangement was found to be especially'effective in reducing corona discharge and radio interference since the liquid dielectric'soaked crep paper substantially eliminated all air in this space.

Another important feature of the crepe-paper wrapping is the fact that it completely fills the space within the bushing and, consequently, prevents reflected gases upon current interruption, such as may be expelled from the fuse tube upon operation thereof, from going up around the outside of the fuse tube to cause flashover. As may be observed from Fig. 2, the small diameter fuse wire 23 is positioned a considerable distance below cover ll. of casing 10 to eliminate high-voltage'stress and, consequently, greatly to reduce radio interference, The flexibl lead 24 has a larger diameter and is surrounded by considerable high-voltage insulation so that the highvoltage gradients or stresses from the lead to the to minimize impulse corona and radio interference.

In Fig. 4, we have shown a modification of the fuse device of our invention which is generally designated as l5 while the corresponding partsthereof are designated by the same reference numerals as the fuse device of Fig. 3. Instead of providing a crepe-paper wrapping for the entire fuse as in Fig. 3, we provide a suitable tube or ferrule 33 of insulating material which may be threadedly mounted to the upper portion of the outer tube ll so as to surround completely the 'upper terminal 88. Preferably, this tube is constructed so as to provide a space surrounding the fuse terminal 18 which may be filled with a suitable insulating compound 34 as indicated, not only to aid in reducing radio interference but also to protect the upper fuse terminal is against arc-over of the fuse tube due to reflected gases upon current interruption. It should b understood that, even vwithout the insulating compound 34, the insulating ferrule 33 will minimize the probability of such arc-over.

The lower portion of the fuse tube comprising members l6 and H is preferably provided with a fiber washer 35 to space the fuse tube within the hollow central portion of bushing l2 and also to prevent gases from entering thi space. The upper portion of the fuse device may be provided with a crepe-paper wrapping 32' similar to the wrapping illustrated in Fig. 3.

With the above-described arrangementsjwe have provided a bushing fuse requiring less space than similar devices heretofore which operates very satisfactorily on low currents as well as high currents due to the particular proportioning of the fuse tube bore and the length thereof and, furthermore, which greatly minimizes radio interference and corona discharges when subjected to high-voltage impulses. I

While we have shown and described our invention in connection with certain specific embodiments, it should be understood that our invention is not limited to the specific details herein illustrated and that changes and modifications may-occur to one skilled inthe art without departing from the spirit and scope of our invention as set forth in the appended claims.

'What we claim as new and desire to secure by Letters Patent of the United States is:

1. An expulsion fuse device for electric apparatus of the type using an insulating dielectric and including a high-voltage bushing comprising a fuse. tube of insulating material having a bore of inchor less, a fusible element positioned in said fuse tube, said fuse tube bore having a length in inches equal to the ratio of the product of the operating volts and the bore in inches to a constant having a value between 200 and 400,

and means for positioning said fuse tube in said bushing.

2. A bushing fuse for electrical apparatus of the type having a high-voltage bushing, comprising a. fuse tube of insulating material having a bore of A; inch or less, said bore having a length.

in inches equal to Vd/K, where V is the operating voltage of the fuse device, d is the bore in inches, and K s a constant having a value falling within the range between 200 and 400, a

fusible element positioned in said fuse tube, and means for supporting said fuse tube within said bushing.

3. An expulsion fuse device for electric apparatus of the type employing an insulating liquid dielectric and including a high-voltage bushing comprising a fuse tube of insulating material, a. fusible element positioned in said tube, means for positioning said fuse tube so as to be completely housed within said bushing, and means for minimizing radio interference at operating voltages and corona discharge when said fuse device.

is subjected to high-voltage impulses comprising a wrapping of absorbent. insulating material capable of becoming saturated with said liquid dielectric so as effectively to surround the main portion of said fuse device with said liquid dielectric even though the main body of said device is located above the level of the liquid dielectric in said apparatus.

4. An expulsion fuse device for electric apparatus of the type including a high-voltage bushing comprising a fuse tube of insulating material, a terminal associated with each end of said fuse tube, a fusible element positioned in said tube and interconnecting said terminals, an insulating ferrule surrounding one of said terminals and adapted to be filled withan insulating compound so as to exclude air from around said terminal to minimize radio interference, and means for positioning said fuse tube substantially within said high-voltage bushing.

5. In an expulsion fuse, the combination of a fuse tube of insulating material having a bore of inch or less, a pair of terminals associated with said fuse tube, and a fusible element positioned in said fuse tube interconnecting said terminals, said fuse tube having a length in inches equal to the product of the operating volts and the bore in inches divided by a constant having a value falling within the range between 200 and 400.

jected to high-voltage impulses comprising a 'wrapping'of absorbent. insulating material capable of becoming saturated with said liquid dielectric so as effectively to surround the main portion of said fuse device with said liquid dielectric even though th main body of said device is located above the level of the liquid dielectric in said apparatus.

7. An expulsion fuse device for electric appara tus of the type employing an insulating liquid dielectric and including a high-voltage bushing comprising a fuse tube of insulating material having a bore of inch or less and a length in inches equal to the ratio of the product of the 6. An expulsion fuse device for electric apparaoperating volts and the bore and a constant having a value falling within the range between 200 and 400, a fusible element positioned in said tube, means for positioning said fuse tube so as to be completely housed within'said bushing, and means for minimizing radio interference at operating voltages and corona discharge when said fuse device is subjected to high-voltage impulses comprising a wrapping of absorbent insulating material capable of becoming saturated with said liquid dielectric so as effectively to surround the main portion of said fuse device with said liquid dielectric even though the main body of said de'- vice i located above the level of th liquid dielectric in said apparatus.

8. An expulsion fuse device for electric apparatus of the type employing an insulating liquiddielectric and including a high-voltage bushing comprising a fuse tube of insulating material, a

fusible element positioned in said tube, means for,

positioning said fuse tube so as to be completely housed within said bushing, and means for minimizing radio interference at operating voltages and corona discharge when said fuse device is subjected to high-voltage impulses comprising a crepe-paper wrapping capable of becoming saturated with, said liquid dielectric so as efiectively to surround the main portion of said fuse device with said liquid dielectric even though the main body of said device is located above the level of the liquid dielectric in said apparatus,

SIDNEY R. SMITH, JR. CHARLES R. CRAIG. 

